Add a new way to record Rappor metrics for MIDI permissions.
Rappor metrics for constant permissions were added, but it was based
on old way that may be deprecated soon. This change adds new way to
record metrics that others use.
Also, rappor.xml is updated that was missed in the last change.
BUG=465661
TBR=mlamouri@chromium.org
Review-Url: https://codereview.chromium.org/2038403002
Cr-Commit-Position: refs/heads/master@{#398770}